In many radio frequency (RF) power amplifier (PA) circuit applications, (such as cellular telephone PAs), it may be desirable to measure the power delivered to the load (antenna) in real time. The power measurement may be used as feedback to adjust the amplifier bias point and/or gain to compensate for varying load and temperature conditions. It may be desirable to compensate a RF PA circuit to maintain a constant output power under varying conditions, such as varying load and temperature conditions, for example. Compensation may provide improved performance at the system level and may ensure compliance with FCC emission requirements, for example.
FIG. 1 illustrates a conventional coupler 100 used in a RF PA circuit for measuring power delivered to a load. Coupler 100 comprises an electrical circuit element 110 that is subject to incident power 116 and reflected power 118 between ports 112, 114. Coupler 100 also comprises a coupled line 120 to couple incident power 116 and reflected power 118 in circuit element 110 between coupled ports 126, 128 of coupler coupled line 120. Coupled line 120 exhibits backward coupling and may be terminated on one end by power detector 122 to measure coupled incident power 130 and may be terminated at another end by power detector 124 to measure coupled reflected power 132. Power delivered to circuit element 110 may be calculated by taking the difference between the coupled incident power 130 and the coupled reflected power 132.
In many applications, coupler 100 may be formed on a commercial circuit board such as a flame retardant 4 (FR4) printed circuit board. In such implementations, coupler 100 is usually large relative to the size of a typical PA circuit and therefore may add unnecessary cost to PA circuitry. The characteristics of coupler 100 may be frequency dependent, i.e., the amount of coupled incident power 130 and coupled reflected power 132 may depend on the coupler's electrical coupling length. Electrical losses of coupler 100 may be proportional to its electrical coupling length. Therefore, because coupler 100 may be long, its losses may be substantial. Accordingly, the performance of a conventional coupler may be frequency dependent due to its electrical coupling length. Those skilled in the art will appreciate that it is difficult to design a coupler with an octave bandwidth to minimize its electrical coupling length and hence to minimize its frequency dependency on its electrical coupling length. Those skilled in the art will appreciate that an octave spread covers a two to one frequency ratio, and accordingly, an octave bandwidth has an upper frequency of operation that is twice the lower frequency of operation. Therefore, in conventional power coupling implementations, coupler 100 requires substantial circuit board surface area, is difficult to design, is circuit specific, and its physical characteristics are substantially frequency dependent.